Hydrogen-permeable structure and method of manufacturing the same

ABSTRACT

A hydrogen permeable structure includes a base material ( 1 ) including porous ceramic, and a hydrogen permeable film ( 2 ) formed on the base material ( 1 ), including palladium (Pd) and at least one element other than palladium and having an amount of hydrogen dissolution at a prescribed temperature smaller than that of palladium alone. The hydrogen permeable film ( 2 ) is formed on the surface of the porous ceramic base by a physical vapor deposition technique after any pin holes in the surface of the base have been filled with a porous oxide material.

FIELD OF THE INVENTION

The present invention generally relates to a hydrogen permeablestructure and a method of manufacturing the same, and more particularly,to a hydrogen permeable structure in which a hydrogen permeable film isformed on a porous base material and a method of manufacturing the same.

BACKGROUND INFORMATION

Hydrogen gas is used as a fuel for a fuel cell and the like, and ismanufactured e.g. by a method of transforming gaseous fuel. Forinstance, according to the method of transforming gaseous fuel, watervapor is reformed to produce hydrogen gas, the reformed gas including,in addition to hydrogen as a principal component, carbon monoxide,carbon dioxide and the like as sub components. If the reformed gas isused as it is for a fuel cell, the cell performance deteriorates. Thus,there is a need for removing sub components, i.e. components other thanhydrogen, to refine the reformed gas in order to obtain a high purityhydrogen gas. One refining method utilizes a characteristic of ahydrogen permeable film that selectively allows only hydrogen to passthrough the film. For use, the hydrogen permeable film is formed on aporous support or base material.

For instance, Japanese Patent Laying-Open No. 11-267477 has proposed ahydrogen permeable structure in which a hydrogen permeable film such asa Pd film, Nb film or the like having a thickness of approximately 0.1to 20 μm is formed by an ion plating technique on the surface of aporous support made of stainless steel or ceramic such as alumina andsilicon nitride.

Moreover, Japanese Patent Laying-Open No. 11-286785 has proposed ahydrogen permeable structure in which Pd metal and metal to be alloyedwith Pd are alternately layered on the surface of a porous support by anelectroless plating technique or the ion plating technique, which issubsequently subjected to a heating process, to form a Pd alloy film asa hydrogen permeable film.

Furthermore, Japanese Patent Laying-Open No. 4-349926 has proposed ahydrogen gas separation film in which silica gel having an average porediameter of 10 to 30 Å, alumina gel having an average pore diameter of15 to 30 Å or silica-alumina gel having an average pore diameter of 10to 20 Å is formed in pores of an inorganic porous body having porediameters in the range between 10 and 10000 Å, and a thin filmcontaining palladium is formed on the surface thereof as a hydrogenpermeable film.

Japanese Patent Laying-Open No. 10-28850 has proposed a hydrogenseparation structure including a base material made of porous ceramic orporous glass, a first layer layered on the base material, and a secondlayer layered on the first layer and made of Pd or a Pd alloy as ahydrogen permeable film, the first layer being formed of a materialhaving a thermal expansion coefficient within the range between that ofthe base material and that of the second layer. The first layer relievesstress applied between the base material and the second layer when thehydrogen separation structure is exposed to an atmosphere with largetemperature variation, to prevent the second layer from peeling off fromthe base material.

Japanese Patent Laying-Open No. 11-267477, Japanese Patent Laying-OpenNo. 11-286785, or Japanese Patent Laying-Open No. 4-349926 discloses astructure in which a hydrogen permeable film is formed on the surface ofa porous support, which has suffered from peeling of the hydrogenpermeable film when the hydrogen permeable structure is used in theatmosphere of various conditions, presenting a problem in durability.

To prevent the hydrogen permeable film from peeling off, the hydrogenseparation structure disclosed in Japanese Patent Laying-Open No.10-28850 has employed a layer, formed of a material having a thermalexpansion coefficient within the range between that of a porous basematerial and that of a hydrogen permeable film, interposed between theporous base material and the hydrogen permeable film.

By merely relieving the difference in the thermal expansion coefficientsbetween the porous base material and the hydrogen permeable film,however, it was difficult to effectively prevent peeling of the hydrogenpermeable film.

SUMMARY OF THE INVENTION

An object of the present invention is, therefore, to provide a hydrogenpermeable structure that can more effectively prevent peeling of ahydrogen permeable film and thereby having increased durability, and amethod of manufacturing the same.

The present inventors have examined various possible causes of peelingof a hydrogen permeable film, and found that the primary cause of thepeeling is the compressive stress occurring due to lattice expansions ofmetallic crystals associated with hydrogen dissolution, rather than thedifference in thermal expansion coefficients between a porous basematerial and the hydrogen permeable film, and that such peeling can beprevented by forming a hydrogen permeable film with a small amount ofhydrogen dissolution.

Based on such findings, the above object has been achieved according tothe invention in a hydrogen permeable structure comprising a base madeof a material including a porous ceramic, said base having a basesurface with at least one pin hole in said base surface, a porous oxidematerial filling said at least one pin hole thereby making said basesurface plane, a hydrogen permeable film including palladium and atleast one element other than palladium on said plane base surface, saidhydrogen permeable film having, at a prescribed temperature, an amountof hydrogen dissolution, which is smaller than an amount of hydrogendissolution in palladium alone at said prescribed temperature.

The above defined hydrogen permeable structure is manufactured accordingto the invention by the following steps:

a) producing a base of a material including a porous ceramic material,said base having a base surface with at least one hole in said basesurface,

b) filling said at least one hole in said base surface with a porousoxide material to thereby make said base surface plane,

c) forming on said plane base surface a hydrogen permeable film made ofa film material including palladium and at least one element other thanpalladium, said film material having at a prescribed temperature, anamount of hydrogen dissolution that is smaller than an amount ofhydrogen dissolution in palladium alone at said prescribed temperature.Preferably the forming step is performed by vapor deposition.

A modified embodiment of the present hydrogen permeable structure ismanufactured by the following steps:

a) producing a base of a material including porous ceramic materialhaving a base surface,

b) vapor depositing on said base surface a hydrogen permeable film madeof a film material including palladium and at least one element otherthan palladium, said film material having, at a prescribed temperature,an amount of hydrogen dissolution that is smaller than an amount ofhydrogen dissolution in palladium alone at said prescribed temperature,and

c) performing said step b) in a vacuum atmosphere of 13.3 Pa at themost, and by applying a potential difference of at least 400 V betweensaid base and a raw material which provides said film material.

The amount of hydrogen dissolution in % by weight is defined as a valuemeasured according to the method described in the EXPERIMENTAL sectionof “Solubility of Hydrogen in Palladium-Silver Alloys” in RussianJournal of Physical Chemistry 47(1) published in 1973, and is based on avalue measured using a bulk sample with the same composition as thehydrogen permeable film.

According to the present invention the hydrogen permeable film has, at aprescribed temperature, a hydrogen dissolution amount that is, smallerthan the hydrogen dissolution amount of palladium alone at the sametemperature. Compared with a conventional hydrogen permeable metal filmof palladium alone, in the working temperature range including aprescribed temperature, the invention reduces the hydrogen dissolutionamount of the hydrogen permeable film thereby reducing the expansion ofthe crystal lattice of the palladium metal and of the film, whereby filmpeeling is suppressed. The compression stress imposed on the film by itsexpansion is reduced, whereby the stress applied at the interfacebetween the film and the base is reduced. This feature significantlyreduces the physical deterioration of the hydrogen permeable filmparticularly peeling, cracking and the like, are reduced and thedurability of the hydrogen permeable structure is improved.

Preferably, the prescribed temperature in the hydrogen permeablestructure, is at least 200° C. and at most 700° C.

More preferably, in the hydrogen permeable structure of the presentinvention, the at least one element other than palladium that isincluded in the hydrogen permeable film is platinum (Pt).

More preferably, in the hydrogen permeable structure of the presentinvention, the hydrogen permeable film includes palladium and platinum,the content of the platinum being at least 5% by mass and at most 15% bymass. Increasing the content of platinum can further reduce the amountof hydrogen dissolution into the film, though it lowers the permeabilityor permeation speed of hydrogen gas through the film. In order toimprove the hydrogen gas permeability to a degree higher than that ofthe hydrogen permeable structure made of palladium alone and to enhancethe durability of the hydrogen permeable structure by reducing theamount of hydrogen dissolution into the film, it is suggested that thecontent of platinum in the hydrogen permeable film, including palladiumand platinum, is preferably set within the range between 5 to 15% bymass.

In the hydrogen permeable structure of the present invention, the porousceramic forming the base is preferably silicon nitride (Si₃N₄). Amongvarious types of ceramic, silicon nitride is superior in strength,fracture toughness, abrasion resistance, chemical resistance and heatresistance, whereby a further enhancement of the durability of thehydrogen permeable structure of the present invention is achieved.

The porous base material including porous ceramic material has at leastone hole in the surface, and a porous oxide material or layer ispreferably applied to fill the hole. Thus, the surface of the basematerial is made plane while the hole at the surface is filled with theporous oxide layer, allowing the hydrogen permeable film to be formed onthe surface of the base material in a closely packed manner without pinholes, whereby the permeability of the hydrogen permeable film isimproved. Moreover, the adhesion between the surface of the base and thehydrogen permeable film is enhanced, thereby further improving thedurability of the hydrogen permeable structure. The porous oxide layeror material preferably includes at least one material selected from thegroup consisting of aluminum oxide (Al₂O₃), silicon dioxide (SiO₂) andzirconium oxide (ZrQ₂). The most preferred porous oxide material isaluminum oxide.

As described above, according to the present invention, peeling of ahydrogen permeable film and physical deterioration such as peeling andcracks have been significantly reduced, whereby the durability of thehydrogen permeable structure has been improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 which is the only FIGURE shows a schematic cross section of ahydrogen gas separation structure as an embodiment of the presentinvention.

DETAILED DESCRIPTION OF PREFERRED EXAMPLE EMBODIMENTS AND THE BEST MODESFOR CARRYING OUT THE INVENTION

As shown in FIG. 1, according to an embodiment of the present inventiona hydrogen permeable structure is produced by forming an alloy film 2containing palladium and an element other than palladium on a porousceramic base 1. The alloy film 2 functions as a hydrogen permeable film.The alloy film has an amount of hydrogen dissolution at e.g. 400° C.which is smaller than the hydrogen dissolution of a metal film formed ofpalladium alone.

Hydrogen dissolution into palladium metal causes the crystal lattice ofthe palladium metal to expand. The volume of the palladium metal isincreased by 2.8×10⁻³⁰ m³ when one hydrogen atom exists in the crystallattice of the palladium metal. This increased value and the amount ofhydrogen dissolution into the palladium metal film cause an expansion ofthe palladium metal film, which expansion is much larger than thethermal expansion of the palladium metal film itself when the hydrogengas separation structure is used at e.g. 400° C. Therefore, thereduction of the hydrogen dissolution into the film rather than thereduction of the thermal expansion of the film itself, lowers the stresseffective in the interface between the base and the film. As a result,the physical deterioration, such as film peeling and cracks, isefficiently reduced whereby the durability of the hydrogen gasseparation structure is substantially improved compared to conventionalstructures with only palladium in the hydrogen permeable film.

The hydrogen permeable film may include as a component any elementsother than palladium that has an amount of hydrogen dissolution at aprescribed working temperature smaller than that of a film formed ofpalladium metal alone. An embodiment of the invention forms the hydrogenpermeable film by adding platinum to palladium. For example, at thetemperature of 400° C., the amount of hydrogen dissolution per 100 g ofpalladium metal is approximately 15 mg for the palladium metal alone,whereas, for a palladium-platinum-based alloy comprised of 90% by massof palladium and 10% by mass of platinum, the amount of hydrogendissolution per 100 g of the alloy is approximately 8 mg, which is lowerthan the 15 mg mentioned above. Moreover, as for the hydrogen gaspermeability, the amount of hydrogen gas permeation is 2.3cm³/cm²/min·cm for the palladium metal alone, whereas it is 2.8cm³/cm²/min·cm for the palladium-platinum-based alloy comprised of 90%by mass of palladium and 10% by mass of platinum, showing improvement inthe hydrogen gas permeability. It is noted that the measurement isperformed under the condition that the temperature is 500° C., thehydrogen pressure on the supplying side is 303.975 kPa (3 atmosphericpressure) and the hydrogen pressure on the permeation side is 0 kPa (0atmospheric pressure).

The hydrogen permeable film may be formed of a single-layer film of analloy including palladium and an element other than palladium, or mayhave a multi-layered film structure constituted by a plurality of layersof the above alloy.

Considering that the hydrogen permeability of the hydrogen permeablefilm is inversely proportional to the thickness of the film, thethickness of the hydrogen film is preferably at most 10 μm, and morepreferably at most 1 μm.

Moreover, it is preferable to form the hydrogen permeable film on thesurface of a porous ceramic base material that has been made plane insuch a manner that holes at the surface are filled with aluminum oxide,silicon dioxide, zirconium oxide or the like, to reduce pin holes in thefilm surface. More preferably, a porous aluminum oxide layer is formedat a hole portion on the surface of the base to make that surface plane.The surface of the hole portion that has the area ratio of 30-70% iscovered by the porous aluminum oxide layer, while ceramic particles areexposed at the surface of other base portions. The hydrogen permeablefilm formed on the surface of such a base material and the base materialadhere strongly to each other. Such adhesion prevents the hydrogenpermeable film from peeling off from the base material when thehydrogen-containing gas is purified, which allows a close structurewithout pin holes, whereby the amount of gas other than hydrogen passingthrough the hydrogen permeable film is substantially reduced. Therefore,hydrogen gas with a high purity can be obtained.

Though the hydrogen permeable film may be formed by any film-formingmethod, a method of physically depositing a film with a vacuum of atmost 13.3 Pa (0.1 Torr), such as an ion plating technique and aspattering technique, is preferably used to form the film. Here, apotential difference of at least 400V is preferably applied between thebase material (or a base material holder) and a raw material for vapordeposition (a target). The application of such a potential differenceincreases energy used when the raw material for vapor deposition adheresto the base material, improving adhesion of the film to the basematerial.

Though there are various types of ion plating techniques and any typethereof may be applied to the present invention, in particular, an arcion plating technique (arc discharge ion plating technique) ispreferably used.

A film including palladium, for example, has an excellent hydrogenpermeability as a hydrogen permeable film. The hydrogen permeability onthe (100) plane of the palladium crystal is, however, lower than that ofthe other crystal planes. Accordingly, a film including palladium isformed such that the palladium crystals are oriented in their (111)planes in order to obtain hydrogen permeability better than that of thefilm without such orientation. According to the manufacturing method ofthe present invention, in a film including palladium that is formed byapplying a potential difference between the base material and the rawmaterial for vapor deposition, the palladium crystals are oriented intheir (111) planes, so that good hydrogen permeability can be obtained.

For the porous ceramic used as a base material of the hydrogen permeablestructure in the present invention, different types of oxides such asaluminum oxide or various types of nitrides such as silicon nitride maybe applied, silicon nitride being the most preferable in terms ofstrength and the like. The silicon nitride preferably includes therein anet-like cavity portion where columnar β-Si₃N₄ crystal particles areintertangled. Moreover, the porosity of the porous silicon nitride basematerial is preferably in the range between 30 and 70%, more preferablyin the range between 40 and 50%. Furthermore, the flexural strength ofthe porous silicon nitride base material is preferably in the rangebetween 30 to 450 Mpa, and more preferably in the range between 200 and450 Mpa.

EXAMPLE 1

A porous silicon nitride sintered body with the average pore diameter of0.3 μm was prepared as a base material of a hydrogen permeablestructure. Particles of alumunum oxide with the average particlediameter of 0.03 μm dispersed in water were applied on the surface ofthe base material and fired at a temperature of 750° C. for one hour.Thus, a hole at the surface of the base material was filled with aporous aluminum oxide layer to make the surface of the base materialplane and to avoid pin holes.

An arc ion plating device was used as a device for forming a hydrogenpermeable film on the surface of the porous silicon nitride basematerial processed as described above. An alloy having a compositioncomprised of 90% by mass of palladium and 10% by mass of platinum, i.e.a raw material for the hydrogen permeable film, was set as a targetwithin a chamber in the arc ion plating device, the base material andthe target being spaced by the distance of 300 mm. The pressure in thechamber in the arc ion plating device was set to 2.66×10⁻³ Pa (2×10⁻⁵Torr) and then the bias voltage and arc current were set at −1000V and80 A, respectively, in order to provide a potential difference betweenthe base material and the target, and the device was operated for 10minutes. Thus, a palladium-platinum alloy film having a thickness of 0.3μm was formed on the surface of the base material.

For the hydrogen permeable structure manufactured as described above, aheat cycle test was performed for 100 cycles at a temperature between400° C. and room temperature in a hydrogen gas atmosphere of 101.325 kPa(1 atmospheric pressure). Subsequent to this test, the film was examinedfor peeling by visual observation and for cracks by electron microscopicobservation, which showed that no physical deterioration of the filmsuch as peeling or cracks was observed. It is noted that the amount ofhydrogen dissolution per 100 g of an alloy having a compositioncomprised of 90% by mass of palladium and 10% by mass of platinum was,when measured by the method described earlier, 8 mg. Moreover, when202.65 kPa (2 atmospheric pressure) of hydrogen gas was supplied whilethe hydrogen gas on the permeation side was set as 101.325 kPa (1atmospheric pressure), the amount of hydrogen gas permeation was 100cm³/cm²/min at the temperature of 350° C.

Further, when the hydrogen permeable structure was used to purifyhydrogen-containing gas at the temperature of 400° C., thepalladium-platinum alloy film showed good hydrogen gas permeabilitywithout peeling off from the base material, allowing hydrogen gas withhigh purity to be obtained.

COMPARATIVE EXAMPLE 1

A hydrogen permeable structure was manufactured as in Example 1, exceptthat metal of palladium alone for a raw material of a hydrogen permeablefilm was set as a target within the chamber in the arc ion platingdevice. The obtained hydrogen permeable structure was subjected to aheat cycle test under a condition similar to that in Example 1. Afterten cycles, the film was examined for peeling by visual observation andfor cracks by electron microscopic observation, which showed thatpartial peeling of the film was observed by visual observation whilecracks were observed on the film by the electron microscopicobservation. It is noted that, for the metal of palladium alone, theamount of hydrogen dissolution per 100 g of metal was 15 mg. The amountof hydrogen gas permeation was, when measured under the conditionsimilar to that in Example 1, 50 cm³/cm²/min.

COMPARATIVE EXAMPLE 2

A hydrogen permeable structure was manufactured as in Example 1, exceptthat an alloy having a composition comprised of 75% by mass of palladiumand 25% by mass of silver for a raw material of a hydrogen permeablefilm was set as a target within the chamber in the arc ion platingdevice. The obtained hydrogen permeable structure was subjected to aheat cycle test under a condition similar to that in Example 1. Theresult revealed that the film was entirely peeled after one cycle of theheat cycle test and completely off the base material. It is noted thatthe amount of hydrogen dissolution was 75 mg per 100 g of the alloyhaving a composition comprised of 75% by mass of palladium and 25% bymass of silver. Because of the peeling, this sample could not bemeasured for the amount of hydrogen gas permeation.

COMPARATIVE EXAMPLE 3

A hydrogen permeable structure was manufactured as in Example 1, exceptthat metal of palladium alone for a raw material of a hydrogen permeablefilm was set as a target within the chamber in the arc ion platingdevice. The obtained hydrogen permeable structure was subjected to aheat cycle test for 100 cycles in atmospheric air of 101.325 kPa (1atmospheric pressure) at a temperature between 400° C. and the roomtemperature. Subsequent to the test, the film was examined for peelingby visual observation and for cracks by electron microscopicobservation, which showed that no physical deterioration of the filmsuch as peeling or cracks was observed.

As described above, comparison between Example 1 and ComparativeExamples 1 and 2 shows that there is a clear relevance between theamount of hydrogen dissolution into the hydrogen permeable film anddurability of the hydrogen permeable structure, and that the hydrogenpermeable structure in Example 1 according to the present invention issuperior in durability. Moreover, in Comparative Example 3, a heat cycletest was performed in the atmospheric air for the hydrogen permeablestructure having a hydrogen permeable film of palladium metal alone, theresult of which shows that heat expansion of the film has a small effecton durability of the hydrogen permeable structure, and that it is theexpansion of the film due to hydrogen dissolution into the film inhydrogen gas atmosphere that mainly lowers durability.

The hydrogen permeable structure according to the present invention issuitable for obtaining hydrogen gas with high purity which can be usedas fuel in a fuel cell and the like.

Although the invention has been described with reference to specificexample embodiments, it will be appreciated that it is intended to coverall modifications and equivalents within the scope of the appendedclaims. It should also be understood that the present disclosureincludes all possible combinations of any individual features recited inany of the appended claims.

What is claimed is:
 1. A hydrogen permeable structure comprising a basemade of a material including a porous ceramic, said base having a basesurface with at least one pin hole in said base surface, a porous oxidematerial filling said at least one pin hole thereby making said basesurface plane, a hydrogen permeable film including palladium and atleast one element other than palladium on said plane base surface, saidhydrogen permeable film having, at a prescribed temperature, an amountof hydrogen dissolution which is smaller than an amount of hydrogendissolution in palladium alone at said prescribed temperature.
 2. Thehydrogen permeable structure of claim 1, wherein said prescribedtemperature is within the range of at least 200° C. to 700° C. at themost.
 3. The hydrogen permeable structure of claim 1, wherein said atleast one element other than palladium is platinum.
 4. The hydrogenpermeable structure of claim 3, wherein said platinum in said hydrogenpermeable film is within the range of at least 5% by mass to 15% by massat the most.
 5. The hydrogen permeable structure of claim 1, whereinsaid porous ceramic of said base is silicon nitride.
 6. The hydrogenpermeable structure of claim 1, wherein said porous oxide materialfilling said at least one hole comprises at least one porous oxidematerial selected from the group consisting of aluminum oxide, siliconoxide and zirconium oxide.
 7. A method for manufacturing a hydrogenpermeable structure, said method comprising the following steps: a)producing a base of a material including porous ceramic material, saidbase having a base surface with at least one hole in said base surface,b) filling said at least one hole in said base surface with a porousoxide material to thereby make said base a surface plane, and c) formingon said plane base surface a hydrogen permeable film made of a filmmaterial including palladium and at least one element other thanpalladium, said film material having, at a prescribed temperature, anamount of hydrogen dissolution that is smaller than an amount ofhydrogen dissolution in palladium alone at said prescribed temperature.8. The method of claim 7, comprising performing said step of formingsaid hydrogen permeable film by a physical vapor deposition technique.9. A method for manufacturing a hydrogen permeable structure, saidmethod comprising the following steps: a) producing a base of a materialincluding porous ceramic material having a base surface, b) vapordepositing on said base surface a hydrogen permeable film made of a filmmaterial including palladium and at least one element other thanpalladium, said film material having, at a prescribed temperature, anamount of hydrogen dissolution that is smaller than an amount ofhydrogen dissolution in palladium alone at said prescribed temperature,and c) performing said step b) in a vacuum atmosphere of 13.3 Pa at themost and by applying a potential difference of at least 400 V betweensaid base and a raw material which provides said film material.